The present invention relates to a wiring structure in a semiconductor device and a method for forming the wiring structure, and more specifically, it relates to a wiring structure in a semiconductor device, which wiring structure includes contacting holes such as a contact hole, a via hole and a through hole or a trench wiring, and a method for forming the wiring structure.
A semiconductor device has a number of contact holes, through holes or via holes (these will be also generically referred to as xe2x80x9ccontacting holesxe2x80x9d hereinafter). Generally, a contacting hole is formed by forming an insulating layer on an impurity-including region, various electrodes or a lower wiring layer (these will be also generically referred to as xe2x80x9clower conductive layerxe2x80x9d hereinafter) formed in or on a semiconductor substrate, or forming an insulating layer on various electrodes or the lower wiring layer formed on a lower insulating layer; forming an opening portion in the insulating layer; and then filling the opening portion with a metal material. With an increase in the integration degree of a semiconductor device, a fine design rule of a semiconductor production process is being employed, and it is an essential object to accomplish a technique of filling an opening portion having a high aspect ratio with a metal material. Further, as a kind of wiring, a trench wiring is practically used which is a technique for forming an insulating layer on a lower insulating layer, forming a trench portion in the insulating layer and then filling the trench portion with a metal material.
With an increase in the integration degree of a semiconductor device, the design rule of a wiring of a semiconductor device keeps on becoming finer, and a signal delay caused by an increase in a capacitance between wirings is a serious problem which inhibits the higher performance of a semiconductor device. As one means for overcoming the above problem, there is known a method in which an insulating layer is composed of a low dielectric constant material, and developments of low dielectric constant materials having a lower relative dielectric constant than a conventional silicon-oxide-film-based material (relative dielectric constant: 4.2) are under way.
The low dielectric constant materials are largely classified into an organic low dielectric constant material and an inorganic low dielectric constant material. Of these, it is said that an organic low dielectric constant material will be a main stream as a low dielectric constant material for materializing a relative dielectric constant of 3 or less required of a semiconductor device to which a design rule of 0.18 xcexcm to 0.13 xcexcm or smaller is applied. The low dielectric constant material is composed of, for example, carbon (C), fluorine (F), oxygen (O) and silicon (Si) as main components, and it is said that due to the contents of carbon (C) and fluorine (F) in particular, the density of the low dielectric constant material can be decreased, and further that the polarizability of molecules per se of the low dielectric constant material can be decreased, so that a low dielectric constant can be materialized.
There is being developed a method for forming a contacting hole and a trench wiring by providing a low dielectric constant material as an insulating layer, which method comprises the steps of forming the insulating layer, forming an opening portion and/or a trench portion in the insulating layer, then, forming a copper (Cu) layer on the entire surface of the insulating layer including insides of the opening portion and/or the trench portion, and then removing the copper layer on the insulating layer. Meanwhile, in the above method, it is required to form a barrier metal layer between the insulating layer and the copper layer for preventing the diffusion of copper atoms into the insulating layer. Tantalum nitride (TaN) is said to be useful as a material for forming the barrier metal layer. However, the barrier metal layer composed of tantalum nitride has problems that it has a high stress and that it shows a low polishing rate during its chemical/mechanical polishing.
There is therefore aggressively developed a method for forming a contacting hole and/or a trench wiring, in which an organic low dielectric constant material is provided for an insulating layer, the insulating layer is formed therefrom, an opening portion and/or a trench portion are formed in the insulating layer, a tungsten layer is formed on the entire surface of the insulating layer including the insides of the opening portion and/or the trench portion according to a chemical vapor deposition method (CVD method), and then, the tungsten layer on the insulating layer is etched back. The above method will be referred to as a blanket tungsten CVD method.
When the tungsten layer is formed on the insulating layer or on the insides of the opening portion and/or the trench portion by the blanket tungsten CVD method, it is required to form a barrier metal layer as an underlayer for the tungsten layer in advance. The reason therefor is that the tungsten layer formed by the blanket tungsten CVD method has poor adhesion to the insulating layer although it is excellent in step coverage. There is another reason that it is also required to prevent the corrosion of the lower conductive layer with a metal fluoride gas such as WF6 which is a process gas for forming the tungsten layer. Further, there is still another reason that since the temperature for forming the tungsten layer by the blanket tungsten CVD method is a relatively high temperature, it is also required to improve the barrier properties for the lower conductive layer.
For the above reasons, the barrier metal layer is formed from, for example, titanium nitride (TiN). In this case, it is preferred to form a titanium (Ti) layer between the barrier metal layer and the lower conductive layer for forming an ohmic contact with the lower conductive layer.
In the formation of the tungsten layer according to a CVD method, conventionally, the insulating layer is heated up to 420 to 470xc2x0 C. The temperature at which the insulating layer is heated for forming the tungsten layer according to the CVD method will be referred to as xe2x80x9cforming temperaturexe2x80x9d hereinafter. However, when poly(aryl ether) of the following formula is used as a low dielectric constant material, and when the forming temperature is set at 420 to 470xc2x0 C., there occurs a phenomenon that the inside of the opening portion can no longer reliably filled with a tungsten layer. 
wherein R is an alkyl group CnH2n+1.
That is, a failure in filling a tungsten layer in the opening portion takes place. The above phenomenon is caused by the thermal decomposition or the pyrolysis of poly(aryl ether). A tungsten layer was formed at a forming temperature of 450xc2x0 C. by a CVD method and analyzed by a secondary ion mass spectroscopy, and FIG. 7A shows the result. For reference purpose, a tungsten layer was also formed at a forming temperature of 375xc2x0 C. by a CVD method and analyzed in the same manner, and FIG. 7B shows the result. When the results in FIGS. 7A and 7B are compared, it is seen that when the forming temperature is set at 450xc2x0 C., the pyrolysis of poly(aryl ether) starts, and as a result, a tungsten layer includes larger contents of carbon atoms and oxygen atoms.
When poly(aryl ether) is measured by a Fourier-transform infrared spectroscopy, an infrared absorption spectrum caused by an ether bond is found around 1200 cmxe2x88x921. FIG. 8 shows the relationship between the heating temperature of poly(aryl ether) and a ratio of reduction in infrared absorption at 1200 cmxe2x88x921. The heating was carried out at each temperature for 1 hour or two hours. FIG. 8 also shows that the infrared absorption at 1200 cmxe2x88x921 based on an ether bond sharply decreases when the heating temperature is set at approximately 400xc2x0 C. or higher, and this data means that the ether bond of poly(aryl ether) is broken at a heating temperature of approximately 400xc2x0 C. or higher. In other words, poly(aryl ether) has a pyrolysis initiation temperature of approximately 400xc2x0 C.
In the blanket tungsten CVD method, therefore, attempts have been made to set the forming temperature lower than 400xc2x0 C., for example, 350xc2x0 C. to 380xc2x0 C. At a forming temperature in the above range, however, gases such as CO, CO2, C2H4 and H2O are released from poly(aryl ether). As a result, the following phenomenon occurs. Process gases used in the blanket tungsten CVD method, such as WF6, SiH4 and H2, are prevented from entering an opening portion and/or a trench portion formed in the insulating layer, so that it is difficult to form a tungsten layer inside the opening portion and/or the trench portion, and the opening portion and/or the trench portion can no longer reliably filled with the tungsten layer.
Further, as shown in FIG. 9, with a decrease in the forming temperature, the tensile stress of a tungsten layer increases. As a result, a phenomenon that the tungsten layer undergoes cracking takes place, and the reliability of the wiring structure decreases.
Further, the barrier metal layer composed of TiN is liable to be oxidized. When the barrier metal layer surface is oxidized, it is difficult to grow crystal nuclei of tungsten when tungsten is grown on the barrier metal layer by the blanket tungsten CVD method, and the crystal growth of the tungsten layer is inhibited. It is therefore required to prevent the surface oxidation of the barrier metal layer to the lowest level possible. For example, there is known a method in which a side-wall of SiN is formed on the inner wall surface of the opening portion to prevent oxygen atoms from reaching the barrier metal layer from the insulating layer. Since, however, the above method involves a problem that the diameter of the opening portion decreases since the side-wall of an insulating material is formed on the inner wall surface of the opening portion or has a difficulty in that it is required to use SiN having a high relative dielectric constant, the above method cannot be said to be a satisfactory solution means when a low dielectric constant material is used for the insulating layer.
It is therefore a first object of the present invention to a method for forming a wiring structure in a semiconductor device, in which an opening portion and/or a trench portion formed in an insulating layer composed of a low dielectric constant material can be highly reliably filled with a refractory metal layer when a contacting hole and/or a trench wiring is formed, and a semiconductor device obtained by the above method.
Further, it is a second object of the present invention to provide a wiring structure in a semiconductor device, which, in addition to achieving the above first object, can prevent a refractory metal layer from undergoing cracking and is free from the inhibition of growth of the refractory metal layer, and a method for forming the wiring structure.
It is further a third object of the present invention to provide a wiring structure in a semiconductor device, which, in addition to achieving the above first object, can reliably prevent the adverse influence caused by the gas released from an insulating layer composed of a low dielectric constant material and is free from the inhibition of growth of the refractory metal layer, and a method for forming the wiring structure.
The method for forming a wiring structure in a semiconductor device, provided by the present invention, for achieving the above first object comprises the steps of:
(a) forming an insulating layer of a low dielectric constant material having a relative dielectric constant of 3.5 or less and a pyrolysis initiation temperature of 400xc2x0 C. or lower on a substratum and then forming an opening portion and/or a trench portion in the insulating layer,
(b) forming a barrier metal layer on the insulating layer including inside(s) of the opening portion and/or the trench portion,
(c) forming a thin layer on the barrier metal layer, and
(d) forming a refractory metal layer on the thin layer to fill the inside(s) of the opening portion and/or trench portion with the refractory metal layer,
the thin layer being composed of a metal or a metal compound which is less easily oxidizable than a material constituting the barrier metal layer (that is, a metal or a metal compound having a higher free energy for forming an oxide of a metal element which is a component of the thin layer than the free energy for forming an oxide of a metal element which is a component of the barrier metal layer).
In the method for forming a wiring structure in a semiconductor device in the present invention, for achieving the above second object, preferably, the thin layer has a compression stress, and the refractory metal layer has a tensile stress. This method for forming a wiring structure in a semiconductor device, provided by the present invention, will be referred to as the method for forming a wiring structure in a semiconductor device according to the first aspect of the present invention for the convenience.
In the method for forming a wiring structure in a semiconductor device according to the first aspect of the present invention, the temperature for heating the insulating layer during the formation of the refractory metal layer is required to be lower than the pyrolysis initiation temperature of the low dielectric constant material constituting the insulating layer. In this case, the value of the compression stress of the thin layer can be controlled by controlling the condition of forming the thin layer, and the value of the tensile stress of the refractory metal layer can be controlled by controlling the condition of forming the refractory metal layer. Preferably, the thin layer is formed by a sputtering method, and the refractory metal layer is formed by a chemical vapor deposition method (CVD method) while heating the insulating layer at a temperature lower than the pyrolysis initiation temperature of the low dielectric constant material constituting the insulating layer. When the thin layer is formed by a sputtering method, for example, the value of the compression stress of the thin layer can be controlled by controlling the forming temperature, the pressure of a forming atmosphere and an inputted direct current power during the formation of the thin layer. When the refractory metal layer is formed by a CVD method, the value of the tensile stress of the refractory metal layer can be controlled by controlling a composition of the process gases, the forming temperature and the pressure of a forming atmosphere.
In the method for forming a wiring structure in a semiconductor device in the present invention, for achieving the above third object, preferably, the step (c) comprises a step of forming the thin layer of a metal or a metal compound on the barrier metal layer by an ionized sputtering method or a long throw sputtering method, and the above step (d) comprises a step of forming a refractory metal layer on the thin layer by a chemical vapor deposition method to fill the inside(s) of the opening portion and/or the trench portion with the refractory metal layer. This method for forming a wiring structure in a semiconductor device, provided by the present invention, will be referred to as a method for forming a wiring structure in a semiconductor device according to the second aspect of the present invention for the convenience. In this case, the temperature for heating the insulating layer during the formation of the refractory metal layer is required to be lower than the pyrolysis initiation temperature of the low dielectric constant material constituting the insulating layer. The above ionized sputtering method refers to a sputtering method in which particles sputtered from a target are passed through a plasma having an electron density of 1010 to 1012 cmxe2x88x923 and the sputtered particles ionized by their collision with the plasma are directed perpendicularly to the insulating layer with a bias voltage applied to the substratum, which method can improve the coverage on the opening portion and/or the trench portion. The above long throw sputtering method refers to a sputtering method in which the distance between the target and the substratum is increased to intensify the perpendicular directionality of the sputtered particles to the insulating layer, which method can also improve the coverage on the opening portion and/or the trench portion.
The wiring structure in a semiconductor device, provided by the present invention, for achieving the above first object comprises:
(A) an insulating layer composed of a low dielectric constant material having a relative dielectric constant of 3.5 or less and a pyrolysis initiation temperature of 400xc2x0 C. or lower, and formed on a substratum,
(B) a barrier metal layer covering an opening portion and/or a trench portion formed in the insulating layer,
(C) a thin layer formed on the barrier metal layer, and
(D) a refractory metal layer formed on the thin layer and filled in the opening portion and/or the trench portion,
the thin layer being composed of a metal or a metal compound which is less easily oxidizable than a material constituting the barrier metal layer (that is, a metal or a metal compound having a higher free energy for forming an oxide of a metal element which is a component of the thin layer than the free energy for forming an oxide of a metal element which is a component of the barrier metal layer).
For achieving the above second object, preferably, the wiring structure in a semiconductor device, provided by the present invention, has a constitution in which the thin layer has a compression stress and the refractory metal layer has a tensile stress. The above wiring structure in a semiconductor device, provided by the present invention, will be referred to as the wiring structure in a semiconductor device according to the first aspect of the present invention for the convenience.
In the wiring structure in a semiconductor device or the method for forming the wiring structure according to the first aspect of the present invention, there can be employed a constitution in which tungsten (W), molybdenum (Mo) or tantalum (Ta) is used as a metal for constituting the thin layer, tungsten (W) is used as a metal for constituting the refractory metal layer, and titanium nitride (TiN) is used as a material for constituting the barrier metal layer, while the metal for the thin layer is the most preferably tungsten (W). Otherwise, there can be employed a constitution in which tungsten nitride (WNx), molybdenum nitride (MoNy) or tantalum nitride (TaNz) is used as a metal compound for constituting the thin layer, tungsten (W) is used as a metal for constituting the refractory metal layer, and titanium nitride (TiN) is used as a material for constituting the barrier metal layer, while the thin layer is the most preferably composed of tungsten nitride (WNx). It is preferred to form a titanium (Ti) layer between the substratum and the barrier metal layer in advance for forming an ohmic contact.
In the wiring structure in a semiconductor device or the method for forming the wiring structure according to the first aspect of the present invention, desirably, the absolute value of a difference between the absolute value of the compression stress of the thin layer and the absolute value of the tensile stress of the refractory metal layer is 2xc3x97109 Pa (2xc3x971010 dyne/cm2) or less, preferably 1xc3x97109 Pa (1xc3x971010 dyne/cm2) or less. Preferably, the absolute value of the compression stress of the thin layer is 1xc3x97108 Pa (1xc3x97109 dyne/cm2) to 2xc3x97109 Pa (2xc3x971010 dyne/cm2), and the absolute value of the tensile stress of the refractory metal layer is 1xc3x97108 Pa (1xc3x97109 dyne/cm2) to 2xc3x97109 Pa (2xc3x971010 dyne/cm2).
A thin layer or a refractory metal layer is formed on a surface of a silicon semiconductor substrate, and the silicon semiconductor substrate is measured for a distortion caused by the formation (curvature of the silicon semiconductor substrate). The compression stress or the tensile stress can be calculated on the basis of the above distortion.
For achieving the above third object, the wiring structure in a semiconductor device, provided by the present invention, preferably has a constitution in which the thin layer is formed by an ionized sputtering method or a long throw sputtering method, and the refractory metal layer is formed by a chemical vapor deposition method. This wiring structure in a semiconductor device, provided by the present invention, will be referred to as the wiring structure in a semiconductor device according to the second aspect of the present invention for the convenience.
In the wiring structure in a semiconductor device or the method for forming the wiring structure according to the second aspect of the present invention, there can be employed a constitution in which a metal or a metal compound for constituting the thin layer is one selected from refractory metals such as tungsten (W) and tantalum (Ta) or metal compounds having a high melting point such as tungsten nitride (WNx) and tantalum nitride (TaNy), while it is preferred to use tungsten as a metal for constituting the thin layer. Further, it is preferred to employ a constitution in which tungsten is used as a metal for constituting the refractory metal layer and titanium nitride (TiN) is used as a material for constituting the barrier metal layer. It is preferred to form a titanium (Ti) layer between the substratum and the barrier metal layer in advance for forming an ohmic contact.
In the wiring structure in a semiconductor device or the method for forming the wiring structure, provided by the present invention, the low dielectric constant material includes poly(aryl ether), a cyclic fluorine resin, polytetrafluoroethylene, aryl ether fluoride, polyimide fluoride, benzocyclobutene, amorphous carbon and organic SOG. A second insulating layer of SiO2 or the like may be formed on the insulating layer. The material for the second insulating layer is not limited to the low dielectric constant material. When the second insulating layer is formed in the wiring structure in a semiconductor device or the method for forming the wiring structure, provided by the present invention, the formation of the barrier metal layer on the above second insulating layer is conceptually included in xe2x80x9cforming a barrier metal layer on the insulating layer (composed of a low dielectric constant material) including inside(s) of the opening portion and/or the trench portionxe2x80x9d.
The substratum may be constituted of, for example, an silicon semiconductor substrate, a semi-insulating substrate, an insulating substrate or one of various lower insulating layers formed on these substrates. The above substrates or the various lower insulating layers may have a lower conductive layer formed thereon or therein. The lower conductive layer includes an impurity-containing region (for example, source/drain regions) formed in a semiconductor substrate or a semi-insulating substrate, various electrodes or a lower wiring layer, and various electrodes or a lower wiring layer formed on the lower insulating layer. A contacting hole can be formed by filling the inside of the opening portion with the refractory metal layer. The contacting hole generically includes a contact hole, a through hole and a via hole. A trench wiring can be formed by filling the inside of the trench portion with the refractory metal layer. Further, an opening portion is formed in the bottom of the trench portion, whereby a trench wiring and a contacting hole connected to the trench wiring can be simultaneously formed. Further, a trench portion and an opening portion are provided respectively, the trench wiring and the contacting hole can be simultaneously formed.
In the present invention, the thin layer is composed of a metal or a metal compound which is less easily oxidizable than a material constituting the barrier metal layer, the thin layer corresponding to an underlayer for forming the refractory metal layer is less easily oxidizable, and there can be therefore avoided the problem that it is difficult to form the refractory metal layer.
In the wiring structure in a semiconductor device or the method for forming the wiring structure according to the first aspect of the present invention, the refractory metal layer having a tensile stress is formed on the thin layer having a compression stress, and the tensile stress of the refractory metal layer is therefore offset, so that the cracking of the refractory metal layer can be effectively prevented.
Further, in the wiring structure in a semiconductor device or the method for forming the wiring structure according to the second aspect of the present invention, the thin layer of a metal or a metal compound is formed on the barrier metal layer by the ionized sputtering method or the long throw sputtering method in which the directionality of sputtered particles are intensified, so that the coverage of the thin layer in the bottom of the opening portion and/or the trench portion can be improved. Further, for example, if a bias is applied to the substratum, the thin layer deposited in the bottom of the opening portion and/or the trench portion is re-sputtered, and as a result, the coverage of the thin layer on the inner wall surface(s) of the opening portion and/or the trench portion can be improved. Moreover, the thin layer is composed of a material which is less easily oxidizable than a material constituting the barrier metal layer. Therefore, not only the oxidation of the barrier metal layer can be prevented when the refractory metal layer is formed on the thin layer by a CVD method, but also the thin layer works as a barrier against gas released from the low dielectric constant material, and the process gases which the CVD method uses cannot be inhibited from entering the opening portion and/or the trench portion formed in the insulating layer composed of the low dielectric constant material. Therefore, the process gases are easily adsorbed on, and dissociated from, the thin layer, so that the opening portion and/or the trench portion can be reliably filled with the refractory metal layer. Further, if the thin layer is composed of the same refractory metal as the refractory metal which is to form the refractory metal layer to be formed on the thin layer, for example, if the thin layer is composed of tungsten, crystal nuclei of tungsten are easily formed on the thin layer, and the opening portion and/or the trench portion can be more reliably filled with the refractory metal layer. The above thin layer can therefore constitute an underlayer for forming the refractory metal layer according to a CVD method.